Human respiratory epithelial cell cultures have been utilized as models for studying effects of putative cocarcinogens and tumor promotors in vitro. This study focused on effects of specific aldehydes and peroxides including acetaldehyde, formaldehyde, benzoyl peroxide, and hydrogen peroxide. Administration of benzoyl peroxide resulted in small cells with increased cytoplasmic density. Benzoyl peroxide and formaldehyde inhibited cell proliferation at low concentrations (56 micromolar and 210 micromolar, respectively) and enhanced ornithine decarboxylase activity. Acetaldehyde had surprisingly little effect on growth rates up to 30 mM. At this concentration it resulted in squamous-appearing cells with depressed DNA/RNA synthesis, depressed aryl hydrocarbon hydroxylase activity, and depressed ornithine decarboxylase activity. Hydrogen peroxide and sodium formate, a by-product of formaldehyde metabolism, caused enhanced aryl hydrocarbon hydroxylase activity. These observations provide useful information in the assessment of the acute effects of these test compounds in human tissues and in the comparison with expected effects observed in animal models.